Some close friends that I met offline basically betrayed me and now I feel terrible. Life was so interesting, I joined a Facebook messenger chat with people from all over the world , we basically learned more about our personalities and poured our hearts out. Then someone came who during discussions if a disagreement arose over the personality theory, not personal , they would privately message others about me. Soon after about 6 months of a very life changing experience, people started acting differently towards me , I then found out that they were slandering my name behind my back. Then after confronting everyone they all took her side and completely invalidated me , saying her behavior was normal, even though she never brought any of her problems to me, so I couldn't fix anything. Worst part is is we got so close we were going to meet in real life and now I feel like I shouldn't even go anymore and even if things changed and resolutions were made , things will never be the same. What advice do you have ?

I'm sorry that that happened, and I'm sorry that I have very little advice. I would assume that such a large scale betrayal would be incredibly jarring. I have minimal experience with even one person betraying me, but in the end, I willingly left him. So, I have no advice except to keep pushing forward. There are still people out there who won't betray you, even though it might seem like they'll be hard to find.
If you need to vent any more, you can do it here.

Is that really what you meant to say? What is the significance of mentioning that you met these friends 'offline'?
I'm old, so maybe I don't get it...

I think it's a reference to the fact that Non met the friends on the Internet, and the fact that they were close to meeting up in real life indicates a more serious level of friendship. The fact that it was offline and not online is sort of a colloquial mistake where it's closer to 'off the line'. I'm guilty of saying it that way.